Alport syndrome is a disease of the basement membrane that results in a progressive glomerulonephritis that is often associated with sensorineural hearing loss and ocular manifestations (anterior lenticonis). The hallmark of Alport syndrome is a thickening and splitting of the basement membranes in the glomerulus of the kidney that is easily visualized by electron microscopy. This phenotype results in a loss of integrity of glomerular filtration that results in hematuria, proteinuria and eventually death. The frequency of the disease is about 1 in 5000. At least two forms of the disease exist, one that is X-linked, and the other inherited in an autosomal fashion. The collagen 4A5 gene has been shown to be the mutated gene resulting in the more prevalent X-linked form of the disease. This basement membrane-specific collagen is one of 6 different collagen chains that comprise the chicken wire-like lattice that form the foundation of all basement membranes. Until recently, the gene(s) responsible for the autosomal form of the disease has eluded us. Two recent reports, however, have solved this issue, identifying mutations in collagen 4A3 and 4A4 as causative (Lemmink et al., 1994; Mochizuki et al., 1994). homozygous mutations (or a compound heterozygote) that destroy the function of the protein for either of these genes results in disease. It is the intent of this proposal to to use homologous recombination in embryonic stem cells to "knock out" the collagen 4A3 gene, and to use clones containing the targeted mutation to derive a mouse model system for the autosomal form of Alport syndrome. This model will be first characterized to determine the degree with which it parallels the human condition, and used to accurately study the molecular nature of Alport disease progression. Fundamental questions about chain association in protomer assembly will be addressed. If the murine disease pathology is similar to that observed in humans, it may provide a fertile testing ground for potential therapies.